Spatial and Temporal Variation in River Otter (Lontra canadensis) Diet and Predation on Rockfish (Genus Sebastes) in the San Juan Islands, Washington

A 2014 paper in the journal Aquatic Mammals examines coastal river otter predation on rockfish at three islands in the Salish Sea.

River otter with crab. Photo courtesy of the SeaDoc Society.

Abstract:

The effectiveness of marine protected areas (MPAs) and their effects on the predators of target species have been a matter of discussion for some time. In the Salish Sea, a number of MPAs protect several species of rockfish (Sebastes spp.), three of which are currently listed under the Endangered Species Act (ESA); however, the role of coastal river otter (Lontra canadensis) predation on rockfish populations is poorly understood. This study describes the scarcely studied diet of river otters in the San Juan Islands, Washington, as a first step in assessing the potential effect of these predators on rockfish. Using frequency of occurrence (% FO), we described coastal river otter diet for San Juan, Orcas, and Fidalgo Islands during the summer of 2008. River otters consumed a variety of both fish and invertebrate species. Fish occurred most frequently in their diet at all three islands, including gunnels (Pholidae) (present in 83.6 to 97.3% of scats), sculpins (Cottidae) (79.5 to 97.3%), and pricklebacks (Stichaeidae) (58.9 to 78.1%). Rockfish were present in 2.7 to 21.9% of river otter scat with the highest occurrence at San Juan Island (21.9%). Scat also contained a higher occurrence of juvenile rockfish vs adult specimens. Although rockfish consumption by river otters at San Juan Island has increased since the summer of 1999, consistent with the establishment of MPAs, we cannot attribute the establishment of MPAs as the cause or address the positive or negative potential effects of river otter predation on rockfish recovery. However, this information may assist future studies that use more modern techniques in assessing these effects on rockfish populations.